How Channel Segregates Originates: The Flow of Accumulated Impurity Clusters in Solidifying Steels

TL;DR

This paper reveals that oxide impurity clusters in solidifying steel induce density-driven flows that lead to channel segregates, providing new insights into controlling this defect in large-scale metal casting.

Contribution

It identifies oxide impurity clusters as the origin of channel segregates and demonstrates their dominant role over traditional thermo-solutal convection, extending classical segregation theory.

Findings

Channel segregates originate from impurity clusters in steel.

Impurity clusters induce stronger flow than thermo-solutal convection.

The study offers a new approach to control segregation defects.

Abstract

The phenomenon, channel segregates (CS) as a result of gravity-driven flow due to density contrast occurred in the solid-liquid mushy zones1during solidification, often causes the severe destruction of homogeneity and even some fatal damages. Investigation on its mechanism sheds light on the understanding and controlling of the formation of solidifying metals,earth's core, igneous rock and sea ice. Until now, it still remains controversial what composes the density contrasts and, to what extent, how it affects channel segregates. Here, we show that in experimental 500kg and 100 ton commercial cast steel ingots CS originates from oxide Al2O3/MnS impurity clusters (OICs) initially nucleated from the oxide (Al2O3) particles, which induce an extra flow due to sharp density contrast between clusters and melt. The results uncover that, as OICs enrich and grow, their driven flow becomes stronger than the traditionally recognized inter-dendritic thermo-solutal convection, dominating the subsequent opening of the channels. This study extends the classical macrosegregation theory, highlights a significant technological breakthrough to control CS, and could quickly yield practical benefits to the worldwide manufacture of over 50 million tons of ingots, super-thick slab and heavy castings annually, as well as has general implications for the elaboration of other related natural phenomena.